By Jove!

Jupiter exerts a
special fascination through its sheer size: it is the largest planet, more
massive than all the others lumped together, and its four giant Galilean
satellites are entire worlds in themselves (one of them, Ganymede, is larger than
the planet Mercury). For more than a year now, the Galileo spacecraft has swung
around Jupiter in a huge, looping orbit, dodging the belts of intense radiation
that surround the planet and relaying back a series of breathtaking snapshots.

The new images offer by far the most detailed views yet of the myriad worlds
of the Jovian system. This journey into uncharted terrain has produced the
inevitable flood of surprises and puzzles and even--in the case of the moon
Europa--some new thoughts on the possibility of extraterrestrial life in our
solar system.

Jupiter's Moons: Io, Europa, Ganymede and Callisto

JUPITER

The Great Red Spot

So far the main planet has filled mainly a supporting role in Galileo's
investigations. A false-color mosaic of
Jupiter's belted clouds shows, for the first time, the changing composition
of the various layers in the planet's atmosphere, which is thousands of
kilometers deep.

But the most remarkable images have centered on Jupiter's
enigmatic Great Red Spot, a seemingly perpetual, hurricanelike disturbance whose winds reach
speeds of 500 kilometers an hour. The motion of the clouds is clearly visible in
a new animation.
Enormous thunderheads,
tens or hundreds of kilometers across, seem to dot the region around the Great
Red Spot, possibly contributing to the flow of energy that maintains the fierce
winds.

CALLISTO

Cratered Callisto

The outermost of the Galilean satellites is Callisto, one of
the most heavily cratered objects in the solar system. The prominent scars on
this moon's face include seven chains of
craters, probably produced by the impact of fragmented comets such as
Shoemaker-Levy 9, which crashed into Jupiter in 1994. Planetary scientists were
shocked by recent Galileo images showing that, at small scales, Callisto is oddly
smooth. "We expected to see shoulder-to-shoulder craters," says Ronald Greely of
Arizona State University, who is part of the Galileo imaging team. "Something
strange is going on here, but we just don't know what it is."

GANYMEDE

Singing Ganymede

Ganymede, the largest
of the Galilean satellites, has a battered surface
much like Callisto's. The Galileo spacecraft discovered that Ganymede is not only
planetlike in size but that it also shares two other attributes with Earth: it
possesses a tenuous oxygen
atmosphere (far too thin to breathe, however) and a magnetic field that
deflects the charged particles that buzz relentlessly around Jupiter.

NASA has represented magnetic data as sounds and the rainbow-colored
spectrogram illustrated. Approximately 45 minutes of observations are transformed
and compressed to 60 seconds. Color is used to indicate wave intensity, red
corresponding to strong waves, blue corresponding to weak waves. Time progresses
to the right and frequency (pitch) increases vertically. An animation of
the spectrogram and the accompanying audio (3.9 MB) can be downloaded from the Galileo web
site.

IO

Pele Erupting

Jupiter's pizza-colored moon Io is the most volcanically active
body in the solar system; tidal stresses created by Io's gravitational
interactions with the other Galilean satellites cook the moon's interior and
power a nonstop sequence of sulfurous eruptions. Galileo is monitoring the
changes on Io's surface and watching for
plumes from the moon's volcanoes.

A sequence of vivid, color-enhanced
images paint a suggestive portrait of Io's restless nature. The images above
detail the changes around a volcano called Pele, as seen by Voyager 1 (left),
Voyager 2 (middle), and Galileo (right). The Voyager frames were taken in 1979
when the two spacecraft flew past Io; the Galileo view was obtained in June,
1996. Note the changes in the shape of the deposits further from the vent while
the radial dark features closer to the vent show little change.

In addition, Infrared pictures show the "hot
spots" corresponding to the active regions. Other detectors have demonstrated
that Io has a huge iron core below its hellish surface.

EUROPA

Ice Ridges

But the satellite that has generated the greatest excitement is ice-covered Europa. For years, scientists
have speculated that the same mechanism that powers Io's volcanoes could warm
Europa's interior, so that a global ocean could lie
underneath the frozen surface. The latest Galileo views strengthen the
argument, offering dramatic evidence that Europa's surface has been reworked by ice volcanoes and
geysers. Remarkably high resolution
images, depicting features smaller than a football field, capture complex
ridges created by the tectonic deformation of Europan ice.

"I would bet that there is an ocean on Europa," says Christopher F.
Chyba of the University of Arizona. In a commentary in the January 16, 1997
issue of Nature, Chyba called Europa "one of two prime candidates for a
second habitable world in our own Solar System." In his new novel 3001,
Arthur C. Clarke imagines life-forms that could have evolved around hydrothermal
vents on Europa. Whether such a scenario is at all possible depends on whether
the putative ocean really exists. Galileo scientists are looking for active ice
volcanism on Europa, but definitive proof may demand a more sophisticated study.
Chyba and others are already at work planning a follow-up mission that would scan
Europa with ice-penetrating radar that could detect liquid water deep
underground.

And if an ocean exists? Turning the X-Files slogan on its head, Chyba
remarks that "it is almost impossible to predict what will be out there."

ABOUT THE AUTHOR(S)

Corey S. Powell

Corey S. Powell is a science writer, blogger and editor living in Brooklyn, N.Y. He is a visiting scholar at New York University's Science, Health and Environmental Reporting Program.

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